The proposed research is focused on the critical importance of protein conformational dynamics and interactions for the transport and delivery of both cognate ligands and therapeutic agents to cells. Molecular therapeutics places a great emphasis on drug candidates that must precisely deliver the active ingredients to their target sites. One potential method to achieve this aim is to harness the existing cellular machinery, particularly those systems that transport molecules into the cell. This research focuses on the dynamic processes in the transferrin/transferrin receptor system. A powerful and sensitive technique employed in this laboratory for the study of protein higher order structure and dynamics is mass spectrometry. We will develop new experimental strategies combining hydrogen/deuterium exchange in solution (HDX) with electrospray ionization mass spectrometry (ESI MS) detection that use an array of gas phase ion fragmentation techniques to characterize protein conformation and dynamics at an unprecedented level of detail. The new HDX MS methods will enable characterization of protein higher order structure at resolution close to the single amino-acid residue level, in order to pinpoint dynamic regions of proteins critical for function. Application of several recently developed fragmentation techniques will advance current capabilities to include proteins that have otherwise eluded such characterization (such as those rich in disulfide bonds), and also enable selection and characterization of specific conformers. We will use these methods to address pressing biomedical questions related to delivery of therapeutic agents to cells. Specifically we will apply our methodologies to decipher the detailed molecular mechanism of protein-protein interaction in the transferrin-transferrin receptor system and its modulation by metals and conjugated therapeutic agents. The changes in protein dynamics of transferrin in the metal-bound and free form, and comparison of its behavior in the extracellular environment versus the endosome are critical to understanding this transport process. This in vitro model will be verified by correlating the conformational and receptor-binding properties of various transferrin-cytotoxin conjugates with their ability to traverse the blood-brain barrier in vivo and accumulate in malignant cells.